Gravity thickeners cut volume cheaply but leave watery slurries. Centrifuges and filter presses push solids into cake—and the economics shift fast.
Clarifier sludge in raw‑water pretreatment routinely starts out thin—often about 1–3% solids—and the choice of thickening and dewatering equipment governs everything from footprint to haul‑off costs. Design guidance and operating data show gravity units deliver ~5–10% total solids (TS, the weight fraction of dry matter), while mechanical systems push that to 15–30% TS and, in batch filter presses, up to 50% TS (Ontario guidelines).
That difference is material: for the same feed sludge, centrifuges or presses can reduce final hauling weight by another factor of ~3–5 compared to gravity alone. A simple ROI framing used in utilities: if disposal costs $100/ton of water removed, taking cake from 10% to 25% TS cuts transported tons by 60% and saves $ per ton removed (U.S. EPA).
Gravity thickening performance ranges
Conventional clarifier thickeners use settling to concentrate solids; influent sludge from primary raw‑water clarifiers is commonly ~1–3% solids. Raw primary sludge can be thickened to ~8–10% TS in static gravity units, but results vary by sludge type and conditioning (Ontario guidelines). Surface overflow rates are kept low to allow settling; solids capture can exceed 90%, but the underflow is still a slurry. Even a modest thickening—say 3% to 6% solids—halves volume (U.S. EPA).
Waste‑activated sludge (WAS, biologically generated solids) behaves differently: gravity thickeners of WAS produce only ~2–3% TS unless aided by coagulants (Ontario guidelines). Gravity‑belt thickeners (GBT, a moving porous belt for drainage) lift that to 4–8% TS (Ontario guidelines). Gravity units carry low energy use but demand large footprints and long residence times; they are most suitable as a first‑stage to cut volume roughly 30–50% before further dewatering. In one polymer‑conditioned clarifier case, solids rose from 0.71% to 4.83% and volume fell by ~80% (Water Magazine). Without polymers, gravity thickeners may achieve less capture and lower solids—typical thickener overflow effluent is often 10–20% solids removed (side note).
Many plants anchor raw‑water pre‑treatment with a clarifier, then decide whether gravity thickening alone suffices or whether to stage in mechanical dewatering.
Decanter centrifuges and energy tradeoffs
Decanter (solid‑bowl) centrifuges use high G‑forces to migrate solids to a rotating bowl wall and scrape them into cake. Typical cake solids are ~15–30% by weight with solids capture ≈95–99% (Ontario guidelines). Basket centrifuges (batch) achieve ~8–20% TS and disc centrifuges (thin‑layer) ~4–10% TS (Ontario guidelines).
Footprints are compact and throughput is high, but energy demand is significant—about ~360 MJ per dry ton of solids processed (dry ton refers to the mass of solids excluding water), and skilled operation is required (Ontario guidelines). Energy costs and maintenance tend to be higher than for filters; advantages include continuous operation and low operator exposure to odors (U.S. EPA).
Belt filter presses (BFP) and capture ranges
Belt filter presses are continuous units that drain by gravity and then press sludge between cloth belts. They typically yield ~10–25% TS cake for mixed (primary + WAS) sludges (Ontario guidelines). Older sources give 12–14% TS for WAS alone (Ontario guidelines). Solids capture is ~85–95% (Ontario guidelines).
Compared with centrifuges, BFPs use less energy and have simpler controls, but require larger footprints and more cleaning. BFP cake is wetter than a centrifuge cake of similar feed, but drier than gravity‑only. Properly conditioned thin sludge (e.g., >3% solids) achieves >15% TS cake on a belt press; in one study, cake solids ranged 10–14% despite varied conditions (IntechOpen). Operating data show belts may require 2–20 lb polymer per dry ton (1–10 kg/t) and achieve ~14–25% TS for primary sludges (U.S. EPA, Ontario guidelines). BFPs handle fluctuations less well than centrifuges, and cloth wear can be an issue.
Plate and frame filter presses
Batch plate‑and‑frame filter presses apply very high pressures and yield the driest cakes: 30–50% TS is typical (Ontario guidelines). This is roughly 1.5–2× wetter than thermal drying, but far drier than centrifuge cakes. Drawbacks include high capital, batch operation (intermittent), heavy equipment, and labor (plate handling and cake disposal). Filter presses capture ~90–95% of solids (Ontario guidelines). Many plants use a hybrid approach: gravity or thickening → centrifuge or belt press → optionally a filter press for maximum dryness before incineration or sale.
Polymer conditioning and dosing control
Chemical conditioning with high‑molecular‑weight polymers—typically long‑chain cationic acrylamides (flocculants that bridge particles)—is now central to thickening and dewatering. Polymers enhance drainage, cake formation, and capture (HWEA, Springer). Typical doses in presses or centrifuges are 2–20 kg active polymer per dry tonne of solids (2–20 lb/ton) (U.S. EPA). Proper dosing can cut capillary water and boost cake solids substantially—for example, from ~8% to over 35% in a filter press with heat/stabilization (U.S. EPA).
In practice, adding polymer typically raises belt‑press cake from ~12% (unconditioned) to ~20% or more (IntechOpen). A German case saw polymer‑flocculated clarifier sludge increase TS from 0.71% to 4.83% (~6.8× concentration) and volume drop by ~80% (Water Magazine). Polymers also improve solids capture: adequate dosing (and pH coagulants if needed) is often required to meet ~95–99% capture in centrifuges (Ontario guidelines).
Overdosing raises costs and can produce sticky cake, driving a shift to automated dosing control with real‑time floc imaging or rheology to trim polymer while maintaining cake quality (Springer, Springer, Water Magazine). Plants commonly deploy dedicated polymer feeds and controls; accurate chemical addition is supported by equipment such as a dosing pump, and polymer chemistry options range from flocculants to coagulants. Overall, polymer conditioning typically reduces final wet volume by tens of percent beyond unconditioned operation, lowering disposal and transport costs (cost reports often cite polymer prices of $1–2/kg balanced by reduced hauling and compliance costs: Water Magazine, HWEA).
Disposal targets and selection criteria
Table 17‑2 in Ontario’s design guidelines encapsulates the typical dewatering ranges: solid‑bowl centrifuge cake ~15–30% TS versus belt press ~10–25% TS (Ontario guidelines). Choice depends on plant size, required dryness, space, and budgets. Required solids concentration for landfill is often >15–20% TS, while incineration objectives push >30% TS for autogenous operation (Ontario guidelines, Ontario guidelines).
In brief: gravity thickening is low‑cost but yields ~5–10% TS, so volume remains high; mechanical dewatering with polymers routinely produces 15–30% TS, and filter presses reach up to 50% TS, with >90% solids capture across these methods (Ontario guidelines). Many facilities combine stages—from gravity to centrifuge or BFP and, where necessary, to a filter press—to meet disposal targets and transport economics.
Sources and design data
Authoritative design and case data underpin these ranges. U.S. EPA and Ontario guidelines supply typical solids and volume outcomes for each method (U.S. EPA, Ontario guidelines). Recent literature and industry casework detail measured outcomes—solids concentration, volume savings, and polymer dosage—including polymer‑aided thickening from 0.71% to 4.83% TS with ~80% volume reduction (Water Magazine), belt‑press performance and conditioning thresholds (IntechOpen), and polymer selection/dosing guidance (HWEA, Springer).
